If a dart falls out of the board before a player has completed their throw, or if it's knocked out of the board by another dart, it doesn't count. Specifically, a dart must have its point touching the board at the completion of the throw to count. A player completes their throw when they remove their darts from the board (technically, when they touch a dart that is in the board).

Imagine a darts match between two players, who we'll call Alice and Bob, because tradition. Both Alice and Bob require double 1. (Let's say it's a post-COLIN darts tournament.) Alice throws her first dart, and it hits the 1. Alice is bust and it's Bob's turn, right? Not necessarily. If that dart were to fall out, it wouldn't count and Alice would have another two darts at double 1.

So imagine this situation. Both players are on double 1. Alice's first dart hits the 1, and it looks like it's loose, but it doesn't fall out. Bob gets ready to take his turn. But then Alice throws her second dart in an attempt to knock out the first one, and is successful. Both darts land on the floor. Alice then throws her third dart, which lands in double 1.

Alice retrieves her third dart from the board and claims she's won - her first dart didn't count because it was knocked out by the second, and so wasn't in the board at the completion of the throw. Bob says this is ridiculous, and everyone knows that once you've bust you can't keep throwing darts. Who is right?

(Before anyone suggests it, I did ask Kirk this at COLIN a couple of years ago. I have completely forgotten what he said.)

The rules (in the link) are not 100% clear, but referee discretion is reserved, and the referee is going to call "no score" before a second dart is thrown. (Anyway it would incredibly difficult to throw a second dart such that it'll knock the first dart out without sticking and scoring itself.)

In an un-refereed match, common sense would suggest it is effectively a "dead ball" after the first dart goes in and is clear it isn't going to fall out without any further external force applied to it.

In a refereed match, the throw would be bust as soon as the ref calls "no score", which he/she should do as soon as the first dart goes in. I guess if the player quickly throws the other dart(s) before "no score" is called and dislodges the first one then technically that would be fine, but the chances of this happening are probably smaller than Elvis Presley walking in mid-match and asking where the toilets are.

You could throw a brick or something and claim its your second dart. So long as the brick is less than 30.5cms in length.

Fair play to the player who's competing at the business end of a leg despite one of his three darts being a brick just in case this scenario arises. I can't help thinking he might be putting himself at a disadvantage in the medium to long term.